A lithium-ion battery should never be:

Lithium-ion batteries store a massive amount of energy in a very compact package and have extremely low internal resistance. If the positive and negative terminals are short-circuited, the battery will rapidly dump all of its stored energy. This sudden, massive release of current generates extreme heat, which can cause the battery to vent toxic gases, catch fire, or even violently explode due to a phenomenon called thermal runaway. While leaving them at room temperature or disconnected is perfectly safe, and recharging them is their primary function, short-circuiting them must be avoided at all costs.